Wireless communication systems have become a major way of communication, replacing conventional land-based communication systems in many applications. There are many types of wireless communication systems, such as, cellular phone systems, wireless local area networks (LAN), Wi-Fi, ad-hoc networks and wireless sensor and control networks used in cyber-physical systems. While wirelessly communicating using a cellular phone system is well known, the other wireless communication systems are growing rapidly.
One common problem frequently encountered in wireless interactions between a receiver and a transmitter is the presence of interfering signals or interference originating from devices other than the transmitter and the receiver. Depending on the type of the wireless communication, in some applications, this interference may be intentional, such as the jamming of military wireless transmissions. In other applications, the interference may be accidental, for example, resulting from multiple users sharing a common wireless channel with or without a base station. The presence of such interfering signals can severely compromise the ability of the receiver to discern the signal from the intended sender, resulting in a reduction, sometimes significant reduction, of information throughput of the wireless transmission from the transmitter to the receiver.
To overcome the problem associated with interfering signals, several techniques, depending upon the type of the wireless communication system, are commonly employed. For example, in a Frequency Division Multiple Access technology (FDMA) multi-user cellular phone system, users are assigned non-overlapping frequency slots by the base station in order to avoid inter-user interference. Similarly in a Time Division Multiple Access technology (TDMA), users are assigned non-overlapping time-slots, while in a Code Division Multiple Access technology (CDMA), users are assigned non-overlapping orthogonal codes. Because of the limitation of the base stations, users that are outside a particular geographical area of the base station may not be controlled by the base station, and hence those users may interfere with the users within that geographical area.
Specifically, data may arrive at the receiver in a sequence of data-blocks. When the data is independent and identically distributed (i.i.d.) (e.g., in cellular phone systems operated in a licensed spectrum), the statistics of each data-block may be the same, although their realizations may be different. The statistics of each data-block may be estimated from past data-blocks and the statistics of future data-blocks may be predicted by the past and present data-blocks. Various i.i.d.-based signal processing techniques, such as, minimum mean square estimations (MMSE) and multi-user detection (MUD) may utilize the statistics described above to effectively control interference signals in i.i.d.-based wireless communication systems.
On the other hand, when the interferences are non-i.i.d., interference signals are typically not well controlled. This is generally due to the fact that the statistics of data-blocks may abruptly change from one data-block to another, and are not related to each other. Interferences in such networks may be significant because part of the spectrum utilized by those networks is unregulated or unlicensed. Examples of non-i.i.d. based systems may include wireless LAN, Wi-Fi and ad-hoc networks. When the wireless communication system is working under an unlicensed spectrum, the interference signals are not necessarily i.i.d. and, therefore, the conventional i.i.d.-based techniques (such MMSE and MUD used in cellular phone systems) are not so effective. These non-i.i.d. techniques are especially not effective when the interference is large. By virtue of utilizing the conventional techniques for overcoming interference, the current technology confines the wireless communications in wireless LAN, Wi-Fi and ad-hoc networks to be in the local area with limited number of users.
Moreover, the total effect of interference signals in cyber-physical systems can also be non-i.i.d. whether they are operated in a licensed spectrum or an unlicensed spectrum. This is because these interferences arrive randomly and hence cannot be synchronized in time. When these signals arrive randomly and cannot be synchronized in time, the total effect of these interferences is non-i.i.d. and cannot be effectively controlled utilizing conventional techniques.
Accordingly, there exists a need for a technique that may be utilized for effectively countering the negative effects of interfering signals particularly, in an unlicensed spectrum where the interferences are non-i.i.d.